In conventional manufacture of structural steel parts for a variety of machinery such as transportation machinery, machinery for industrial use, construction machinery, and the like, such steel parts are generally either (a) formed roughly into predetermined shapes through hot working, then formed into desired shapes through machining, followed by thermal refining through quenching and tempering, or (b) subjected to hot workings and then quenching and tempering, followed by machining.
However, as structural parts for machinery have been improved to be of high strength, the cost for machining has been increased accordingly. Therefore, for ease of machining and for lowering the costs there is an increased demand for free cutting steel having excellent machinability.
It is well known that the machinability of steel is improved through addition of free-cutting elements (machinability-improving elements) such as Pb, Te, Bi, Ca and S, singly or in combination. For this reason, in order eggs to improve machinability of steels such as steels for machine structural use, there has been employed the method of incorporating the above free-cutting elements into the steels. However, when the free-cutting elements are merely incorporated into steels for machine structural use and the like, in many cases the desired mechanical properties (for example, toughness and fatigue strength) cannot be secured.
Under these circumstances, a technique comprising hot working and then machining, followed by quenching and tempering, as described in (a) above is disclosed in Patent Application Laid-open (Kokai) Nos. 2-111842 and 6-279849. This technique involves "hot rolled steel products endowed with excellent machinability and hardenability" in which C is present in steel as graphite and the machinability of the steel is improved through utilization of the notch effect and lubrication effect of graphite; as well as the "method of manufacturing steels for machine structural use with excellent machinability."
However, in the steel products disclosed in Patent Application Laid-open (Kokai) No. 2-111842, it is essential that B be incorporated into the steel so that boron nitride particles (BN) serve as nuclei for precipitation, to thereby facilitate graphitization, and thus the steel becomes susceptible to cracks when solidified. In contrast, in the method disclosed in Patent Application Laid-Open (Kokai) No. 6-279849, graphitization in steel is accelerated under the as-hot-rolled condition, through addition of Al and through limitation of O (oxygen) content in steel to a low level. This method requires more than five hours for treatment of graphitization after hot rolling, and thus is not very economical.
In contrast, a technique comprising hot working, and then quenching and tempering, followed by machining, as described in (b) above is disclosed, for example, in Patent Application Laid-open (Kokai) No. 6-212347. This involves "hot forged steel products having high fatigue strength and a method of manufacturing the same" in which steel having a specific chemical structure is quenched immediately after hot forging, followed by tempering, to thereby precipitate TiC. However, in the hot forged steel products obtained by this method, the ratio of N to Ti (N/Ti) is merely specified as less than 0.1, and therefore excellent machinability cannot always be secured. Briefly, if the content of N in steel containing 0.01 to 0.20 wt. % of Ti is merely specified such that N/Ti is less than 0.1, hard TiN may often be formed in a great amount, causing degradation of machinability, and further causing degradation of toughness.
In TETSU-TO-HAGANE (vol. 57 (1971) S484), it is reported that machinability may be improved through incorporation of Ti into deoxidation-adjusted free-cutting steel. However, this publication also describes that incorporation of a great amount of Ti produces a great amount of TiN, resulting in incresed wear of tools and disadvantages in terms of machinability. For example, the life of the drill to a steel having the following composition based on % by weight, C: 0.45%; Si: 0.29%; Mn: 0.78%; P: 0.017%; S: 0.041%; Al: 0.006%; N: 0.0087%; Ti: 0.228%; O: 0.004%; and Ca: 0.001%, is adversely short, and therefore, machinability of above-mentioned steel is poor. Consequently, it is concluded that machinability of steel is not improved through simple addition of Ti.